Within the Science Foundation Priority Program “Physics of the Interstellar Medium” we seek a highly capable student to compare the statistical properties of large scale mapping observations of the ISM in different tracers with hydrodynamic and magneto-hydrodynamic turbulence simulations.

The projects focuses on the application of algorithms identifying coherent structures in maps of complex, turbulent, and filamentary molecular clouds. The successful applicant will investigate the statistical properties of structures found by clump finding algorithms, dendograms, principle component analyses, and similar tools. As a significant observational component, supplementing new mapping observations are to be obtained using the NANTEN2 telescopes in the Atacama desert in Chile.
The applicant will closely collaborate with the members of the KOSMA group and partners in the Priority Program.

The successful applicant will possess a degree in physics or a related  field with a first-class academic record, strong skills in spectroscopy, astronomical line observations or the handling and interpretation of large data sets, and a sound knowledge or, at least, a keen interest in astronomy.

Additional information on the subject and the research of the group can be found at the project webpage. To apply, please send a letter of application including a cover letter
illustrating their suitability for the position of interest, a curriculum vitae, full academic records such as bachelor and master degrees as well as a letter of reference to PD. Dr. Volker Ossenkopf, Universität zu Köln, I. Physikalisches Institut, Zülpicher Straße 77, 50937 Köln.

We encourage electronic applications via email to: 
ossk@ph1.uni-koeln.de

The application is open until the position is filled.

In the case of similar qualification, competence, and specific achievements, women will be considered on preferential terms within the framework of the legal possibilities.  Handicapped candidates with equivalent qualifications will be given preference.
 

There currently is no funding for PhD positions, if you come with your own funding, feel free to contact me and a suitable topic in my research interests can be found.

Scaling Terahertz waveguides to frequencies above 2 THz hits the limit of conventional micromachining. A 1.9 THz waveguide is only 60x120 microns. A possible solution is Silicon micromachining with Deep Reactive Ion Etching with the Bosch process. Another possible solution is the use of 3D photolithography to define the negative of the waveguide, which is metallized afterwards and subsequently dissolved. 

Scaling Terahertz waveguides to frequencies above 2 THz hits the limit of conventional micromachining. A 1.9 THz waveguide is only 60x120 microns. A possible solution is Silicon micromachining with Deep Reactive Ion Etching with the Bosch process. Another possible solution is the use of 3D photolithography to define the negative of the waveguide, which is metallized afterwards and subsequently dissolved.

The dimension of the waveguides is then only limited by photolithography, so that the structures can be almost arbitrarily small. In addition, more complicated geometries become possible such as waveguide couplers. With this technology, balanced HEB Terahertz mixers are feasible which would be ideal candidates for focal plane receivers (e.g. STAR on SOFIA). Their advantage is a simple and efficient coupling of the local oscillator power and their noise suppression for LO noise.

The thesis work will deal with the process development of Silicon DRIE with the ICP180-etcher in our microfabrication lab as well as the RF design of (components of) waveguide mixers, and their tolerance analysis.

Many applications of heterodyne technology in astrophysics need a large instantaneously (i.e. without tuning any components) observable spectral range (intermediate frequency (IF) bandwidth). The present generation of SIS mixers usually allows only 4 GHz instantaneous IF bandwidth. A bandwidth of 18 GHz would open up new areas of application, such as in cosmology or atmospheric physics. The challenges of this work lie in the details of the mixer on the intermediate frequency end. This includes the detector design itself (SIS on Silicon membranes), the optimization of the intermediate frequency impedance match, an integration of the first cryogenic amplifier stage and much more.

(Within a projected 345 GHz/490 GHz array receiver on APEX, a continuation of Radionet, AMSTAR FP6 work)

In a collaboration with Max-Planck-Institut für Radioastronomie in Bonn we are developing a 9-pixel heterodyne receiver for 345 GHz with an expected upgrade with 19 pixels at 490 GHz in a later stage. Initially, the mixers will be conventional double side band mixers. Part of the thesis work will be the RF design and the assembly of the mixers for 345 GHz. Another part will deal with the design and development of side band separating mixers for 345 and 490 GHz. This includes RF design of superconducting mixers, design of compact waveguide mixers for the application in multipixel receivers, RF tests of the mixers and participating in the commissioning of the receiver at APEX in the Atacama desert. Obviously there is a close connection of the project to the detector fabrication in our microfabrication clean room.

Low loss integrated tuning structures are crucial to the RF performance of the THz SIS devices developed in SIS 1-2 THz (1). They need to be adapted to the processing techniques and materials that are used to fabricate the tunnel junctions and to the processing tolerances. In addition, because SIS junctions use more local oscillator power than SHEB mixers, a solution (preferably in the waveguide mixer and not optically) must be found to feed a moderate size THz array uniformly with sufficient local oscillator power, given the existing LO sources. The technical realisation of this design should be possible with the techniques developed for the 5 THz SHEB waveguides.

This thesis work contains RF design work both in waveguide structures and in superconducting transmission lines. In addition RF measurements to test design and/or fabrication and their analysis are a substantial part of the work.

Recent material technology developments open up the possibility of the extension of SIS mixing performance up to approximately 2 THz. Contrary to SHEB mixers, SIS mixers have a virtually unlimited IF bandwidth, and also a proven stability in telescope receivers. The most important step towards this goal is the development of high gap frequency, high current density SIS junctions. To achieve this, as a first step high quality and high current density AlN tunnel barriers are developed on standard niobium thin films (see diploma/MA theses). Subsequently these barriers will be combined with high gap frequency thin films (like NbTiN) instead of niobium to fabricate SIS junctions usable up tot 2 THz. In addition these SIS devices then need to be fabricated on membranes, to be able to use them in waveguide mixers.

This thesis work will deal mainly with process development and device physics in the analysis of DC measured IV curves of the fabricated SIS junctions. In case of a successful development first RF measurements will be done in cooperation with theses work SIS 1-2 THz (2).

For the heterodyne receivers on SOFIA we will mainly use superconducting Hot Electron Bolometer (SHEB) mixing devices. Above about 1.2 THz the only sensitive heterodyne detectors are presently SHEBs. Bolometers can only be used as frequency mixers if their thermal response time is in the pico second range. The inverse of the time constant determines the maximum IF bandwidth. This time constant/IF bandwidth is mostly given by the superconducting material of the bolometer, the substrate and their respective match. Presently, SHEBs usually achieve only 3 GHz bandwidth, whereas 6 GHz is practically indispensable for e.g. extra galactic astronomical observations.

Needed are an optimization of the fabrication of ultra-thin superconducting films (using equipment of our microfabrication lab), new ideas for the film-substrate interface and modelling of the bolometer to learn more about the physics of the HEB mixers. Furthermore the matching to the low noise IF amplifiers and its influence on the IF bandpass need to be studied.